Owing to the prevalence of halogen lamps, low voltage illumination is becoming increasing popular and offers the advantage of small bright lamps together with increased safety in the event of contact with the supply terminals. In particular, the use of low voltage lends itself to track lighting and cable lighting systems, using halogen lamps or other low voltage lamps, which can be moved along a fixed track mounted on the ceiling so as to be directed to those places where illumination is most required.
Various proposals for low voltage illumination are known in the art. Most employ a transformer for transforming the high electrical supply voltage (typically 110 V or 220 V) to a low voltage in the order of 12 V. Since the size of the transformer is dependent on its power rating, such transformers are necessarily bulky. It is therefore known in the field of a.c. illumination to invert the incoming electrical supply voltage using a conventional rectifier and chopper circuit so as to obtain a pulsating a.c. voltage source having a high frequency in the order of 30 KHz. The use of high frequency permits the size of the transformer to be greatly reduced.
In one known arrangement a central transformer provides power at low voltage (typically 12 V) to a track, rail or cable system to which low voltage lamps can then be directly connected. However, it has been found that such a proposal, although attractive, is subject to the drawback that, at high frequency, the tracks operate like a transmission line, radiating energy. This problem is exacerbated as the length of the track is increased, when the inter-conductor gap is increased and when the current flow is increased. Such drawbacks would clearly be overcome by employing a low d.c. voltage source for feeding the tracks, but this is subject to its own problems, in particular relating to the efficiency of rectification.
The conventional manner to produce d.c. voltage from an incoming a.c. voltage source is to rectify the a.c. voltage using a passive rectifier employing conventional bipolar rectifier diodes. Such diodes are typically made from silicon having a forward bias voltage of 0.7 V. Thus, the power dissipated by each rectifier diode during the half cycle that it conducts is equal to the product of the forward bias voltage of 0.7 multiplied by the current passed by the diode. In a typical arrangement for energizing a series of low voltage halogen lamps having a combined current consumption of 25 A, the power loss across each rectifier diode would thus amount to 17.50 W which, when combined with the losses in the chopper circuit and step-down transformer, is unacceptable.
In this regard it is to be noted that inefficient rectification of the output not only increases operating costs but also results in heat dissipation requiring that the physical size of the power supply be increased. This, of course, militates against the very reason for using high frequency in the first place: namely to reduce the size of the power supply. Therefore the conventional approach to rectification, which is widely applied in other systems, is not suitable for halogen lighting due to the use of low voltages and hence high currents and the necessity of keeping power losses down so as to enable a small physical size.
In one popular arrangement, the low-voltage power is applied to two uninsulated conductors in the form of cables or rigid rails to which the lamps are attached. Such an arrangement is subject to the hazard that an electrically conductive short, particularly a thin wire, which is accidentally applied between the two conductors may conduct potentially large currents. The resultant heating of the wire constitutes a fire hazard. This situation manifests itself as an increase in current which may either be detected using a fuse which bums out when the heating effect of the current flow exceeds an allowed threshold, by a heat-sensitive electronic device, or by a more sophisticated current sensing element which is more directly responsive the current flow. For example, U.S. Pat. No. 5,523,653 discloses a low voltage lighting fixture connected to an isolation step-down transformer. The fixture is protected from limited or maximum short circuit conditions by monitoring the secondary current of the step-down transformer until a fault is detected, whereupon the protection circuit de-energizes the primary of the transformer.
None of these solutions is entirely satisfactory because none operates instantaneously when the power rating of the load connected to the supply exceeds the power rating of the supply itself. Specifically, even in the more sophisticated case where current itself is monitored, since the supply voltage is alternating, the current must climb from zero to the danger threshold before the protection element can operate. Even in this brief time interval during the 50/60 Hz cycle, the electrically conductive short can reach dangerous temperatures.